1. Field of the Invention
The present invention relates to an image information detecting apparatus used for a color image forming apparatus such as a copier, a printer, or a facsimile machine to which an electrophotographic process or an electrostatic recording process is applied, and more particularly, to an image information detecting apparatus for detecting a position of a pattern.
2. Description of the Related Art
In a conventional image forming apparatus for obtaining a multiple color image, in general, images having different colors are formed in multiple image forming sections, paper is transported by a transport unit such as a conveyor belt, and the images are transferred onto the paper and overlapped each other to thereby perform multiple color image formation. In particular, in a case of obtaining a full color image by performing multiple color developing, even a slight overlap positional deviation deteriorates image quality. For example, in a case of 400 dpi, even an overlap positional deviation of a fraction of 63.5 μm corresponding to one pixel appears as a change of a color misregistration or a color tint variation and significantly deteriorates an image.
In the initial stage of development of a color image forming apparatus, the multiple color developing is performed by optical scanning with a single image forming section, that is, the same optical characteristic, thereby alleviating the overlap positional deviation of images. However, this method has a problem of requiring much time to output a multiplexed image or a full color image.
In order to solve this problem, there is a method of forming images by different optical scanners so as to separately obtain images of respective colors and overlaying the images of the respective colors one on another on the paper transported by a transport section. However, this method raises a concern about a color misregistration caused when overlaying the images.
Therefore, an image information detecting apparatus for detecting the color misregistration is proposed. In the image information detecting apparatus, a beam emitted from a light emitting surface of a light source unit is condensed by a condensing part and an illumination lens, and a pattern for position detection depicted on an image bearing member being a transferring belt is illuminated with the condensed beam via dustproof glass. Then, specular reflection light from the image bearing member and the pattern depicted thereon is detected by a light receiving unit via a stop and an imaging lens. An image forming section (image forming unit) is controlled to output images of respective colors according to detection signals detected by the light receiving unit.
In order to obtain an image with high accuracy, it is necessary to also refer to accuracy of the image information detecting apparatus. One of the causes for deterioration of the accuracy of the image information detecting apparatus is noise present in a detection signal. As a method for improving a signal-to-noise ratio of the detection signal, there are a method of increasing a received light quantity and a method of reducing noise.
For example, Japanese Patent Application Laid-Open No. 2001-092195 discloses a case of improving the signal-to-noise ratio of the detection signal by adjusting an exposure time. Japanese Patent Application Laid-Open No. H07-036244 discloses a case of improving the signal-to-noise ratio of the detection signal by containing the light receiving unit inside a casing made of metal to thereby reduce noise.
Further, as unit for improving the signal-to-noise ratio of the detection signal, there is a method of illuminating the pattern on the image bearing member with high brightness by selecting an LED having a large optical output or selecting an LED having a high directivity.
In addition, as a method of illuminating the pattern on the image bearing member with high brightness, there is a method of using critical illumination in which the light emitting surface and the image bearing member have a substantially conjugate relationship. In the critical illumination, a small region near the pattern is illuminated in comparison with the conventional method of illuminating a large region including the pattern, which allows light emitted from the light emitting surface to illuminate the pattern with high efficiency.
However, the image information detecting apparatus using the critical illumination has a problem that position detecting accuracy is liable to deteriorate at an occurrence of a positioning error of a constituent element or a positioning error of the image information detecting apparatus itself. If color misregistration is corrected by using such an image information detecting apparatus, a color image forming apparatus that exhibits color misregistration correction performance with stability cannot be provided due to an individual difference of the image information detecting apparatus or an environmental change.